The Walters meteorite

FIELDIANA . GEOLOGY

Published by
CHICAGO NATURAL HISTORY MUSEUM

Volume 10 September 24, 1962 No. 37

THE WALTERS METEORITE

Sharat Kumar Roy>

Chief Curator, Department of Geology

Jewell J. Glass

United States Geological Survey

Edward P. Henderson

United States National Museum

The main mass of the Walters meteorite and the data relating
to its fall are in the Geological Department of the United States
National Museum. During a trip there Dr. Roy had studied the
surface features of the main mass and examined the records of
the fall. Prior to this, in conjunction with his studies on the Para-
gould stone (Roy and Wyant, 1955), he had worked on the interior
structure and examined the minerals in thin sections of the Walters
meteorite from representative specimens in possession of the Chi-
cago Museum. He did not know then that Mr. E. P. Henderson
of the United States National Museum and Miss Jewell J. Glass of
the United States Geological Survey were also making a study
of this meteorite and that their studies on certain of its essential
features were nearing the final stage. To avoid further duplication
of efforts, the results of their investigations were incorporated with
those attained by Dr. Roy, and the manuscript in its present form
was prepared for publication under joint authorship.

CIRCUMSTANCES OF THE FALL

The Walters meteorite fell at 3:45 P.M., July 28, 1946, about
113^ miles west of Walters, in Cotton County, southwestern Okla-
homa (in northeastern corner of NW. 34 Sec. 25, T. 2 S., R. 13 W.;
Lat. 34'' 22' N., Long. 98° 31' W.). The weight was 22.33 kilograms.

The meteorite was purchased by the United States National
Museum from Mr. Frank Moore, Route 5, Walters, Oklahoma.

» Deceased April 17, 1962.

Library of Congress Catalog Card Number: 62-21 66S

No. 958 689

640 FIELDIANA: GEOLOGY, VOLUME 10

Chicago Natural History Museum has a large polished slice of the
main mass. The description of the interior structure is based upon
this slice and that of the surface features upon the main mass.

SHAPE, SURFACE MARKINGS, AND CRUST

The Walters stone is roughly equidimensional. It has many
surface irregularities and its general form lacks the features of
atmospheric shaping of a well-oriented meteorite. The irregularities
indicate that parts of the meteorite had broken off shortly before
it reached the earth and that there had not been time enough for
the broken areas to be reshaped. They can be recognized by their
angularity and partial crusting.

The apex is not well developed but it can be determined from
the smooth and sloping surface immediately surrounding it. The
sloping surface is interrupted by a concavity with broad, shallow
pits — a feature that commonly develops on the rear side where
less air is encountered. Contrariwise, the rear side of the meteorite
shows certain characteristics usually found on the front side, such
as the presence of an incipient apex in the form of a rounded pro-
tuberance, from which have passed currents of air radially that
have given rise to smaller and deeper pits.

At one place, a little above and opposite the apex, the surface
is marked by a V-shaped cleft 4 inches long and 13^ inches deep.
Its walls are coated with slaggy crust, and its edges slope inward
and are rounded. All these suggest that originally the cleft was
a narrow crack that has been enlarged and deepened by the passing
of air currents. The cleft, in turn, is almost entirely bounded by
a fissure, which has weakened the encircled area to the breaking
point, and doubtless it would have broken off had the meteorite
stayed in the air for a fraction of a second longer. To judge from
the damage suffered, it would seem that the meteorite was sub-
jected to considerable shock during its flight.

With the exception of a portion of the apical region, the entire
surface of the meteorite is pitted, but the pits do not conform to
the usual pattern, which would indicate that the mass has en-
countered currents of air from different directions at different times.
The shape and size of the individual pits vary but little from those
of other stone meteorites. Some are circular, some are oval, others
are elongated. The circular ones are the shallowest and the elongated
ones the deepest.

ROY, GLASS, AND HENDERSON: WALTERS METEORITE 541

The arrangement of these pittings, the markedly irregular form
of the meteorite, and the anomalous shaping it has undergone,
strongly indicate that the Walters meteorite did not maintain a
fixed position during its passage through the earth's atmosphere;
it had turned around, in particular, from front to rear near the
end of its flight.

It has been stated and mathematically shown that the apex
of a meteorite is generally in line with the center of gravity of
the mass. Accordingly, if the distinguishing features of the front
or the apical region in a meteorite are found on more than one
of its sides, it can be assumed that the meteorite has changed
positions during its flight. The change is most likely to occur when
a portion of the falling meteorite suddenly breaks off, incurring
loss of weight of the mass and subjecting it to the resultant force
of the break. These are factors that may very well affect the
course of the meteorite and cause it to turn and take up a position
in line with the center of gravity of the reduced mass. Under
these circumstances, the degree of development of the front will
depend on the altitude at w'hich the breaking takes place and on
the speed of fall of the meteorite thereafter; the higher the altitude
and the lower the velocity the longer the time for more complete
shaping.

The crust of the Walters meteorite is black with scattering brown-
ish stains resulting from oxidation that has occurred since its recov-
ery. The surface of the crust, as seen under magnification, is covered
with narrow, thread-like ridges, some of which are straight, some
wavy, others forking. This sort of pattern, formed by fused matter
flowing longitudinally and sending off branches, is a common feature
of the crust of stony meteorites. Areas of contraction cracks and
scoriaceous texture with characteristic pits of expanded gas bubbles
have also been observed. Thickness of the crust varies; the varia-
tion was caused by the partial disruption of the meteorite during
its flight, as different surfaces were exposed for different lengths
of time. On uniformly exposed surfaces the average thickness is
± 0.5 mm. It is only in the fissures and certain angular pits where
the fused matter has accumulated that the crust is perceptibly
thicker, often massive. A noteworthy feature of this meteorite is
the total absence of protruding grains of nickel-iron on the sur-
face of the crust.

Examination of thin sections of the crust shows only one zone —
the outer fused zone. The so-called absorption and impregnation

542 FIELDIANA: GEOLOGY, VOLUME 10 ^

zones are not present, or at least are not recognizable in the sections
examined. Absence of these zones — believed to be formed by-
deeper penetration of heat during flight — in a meteorite so ex-
tensively veined as this one, seems unusual, especially to those who
believe that veins are produced by fused matter from the surface
which flowed into fissures, rather than by the penetration of heat
which fused the walls of the fissures or by the injection of molten
matter into the fissures. A brief discussion of this subject is given
(p. 544).

Another feature observed in relation to this question of heat
penetration is the existence of minute grains of at least two minerals
within the crust just below its outer margin, one of which is olivine,
the other probably hypersthene; the latter has not been positively
identified optically. X-ray powder analyses of three samples of the
crust show the presence also of plagioclase and magnetite. The
existence of these silicates indicates that the surface was not
completely fused to form the crust, and that some of the more
refractory silicates survived the heat.

SIZE AND WEIGHT

Since the acquisition of the meteorite by the United States
National Museum (USNM 1430), several sHces have been cut from
it for distribution and for petrographic and chemical analyses. One
of these slices, as indicated elsewhere, was secured by exchange
and is in the collection of Chicago Natural History Museum (CNHM
Me 2422). At the present time, the approximate measurements of
the main mass of the Walters meteorite are: height 93^ inches;
length 103^ inches; and width 8^^ inches. Weight of the original
mass was 28.33 kilograms.

INTERIOR STRUCTURE

The stone is compact and takes a good poMsh. A cut and polished
section reveals that the most conspicuous feature of this stone is
the presence of numerous black veins. Almost as conspicuous a
feature is the occurrence of countless troilite bodies. Modal analysis
gives 10.4 ± 1 per cent of troihte by volume. So numerous are
these two elements that they have visibly modified the original
homogeneous gray color of the interior to a variegated one of black,
shades of gray, and bronze — the black from the veins and the
bronze from the troilite bodies. Reddish brown stains resulting

ROY, GLASS, AND HENDERSON: WALTERS METEORITE 543

from the oxidation of the nickel-iron grains have spread through
much of the mass and have further modified the original color.

The veins vary in thickness from 0.01 mm. to 10 mm. or wider;
the more numerous ones average 0.1 mm. Some of the widest
and massive ones are localized; they appear more like swellings and
knottings than veins. The course of the veins is usually straight
but it may be curved or undulating. In some areas, two systems
of veins cross at an angle of approximately 90°, forming rectangles;
in others, the veins branch and anastomize and appear like matted
hair. In the light-colored groundmass, the veins may cut through
or surround rectangular or rounded areas and give the interior a
brecciated appearance. In fact, the stone was brecciated earlier
in its cosmic history. Whether the veins cut through or surround
given areas, they merely occupy the fracture lines of a brecciated
meteorite and thus emphasize the shapes and sizes of the fragments
and the nature of the brecciation. This phase has been briefly
elaborated (see p. 544). As in the case of the groundmass, some
of the narrow veins cut across a few larger chondrules, but the
general tendency is to surround them. Here again the veins occupy
the fracture lines or the lines of weakness. The majority of the
chondrules are fractured or distorted. Those that are intact are
not firmly embedded in the groundmass and are commonly char-
acterized by an encircling line of weakness between the two formed
by the shrinkage of the chondrules during their crystallization.
Veins cutting across small chondrules are rare, but a number of
small chondrules are found as inclusions in the veins.

Many of the veins appear to be definitely related to the crust,
and this suggests that they might have originated from fused surface-
matter that flowed into fissures. Some of these have started as
thick flows; others, which were narrow at the beginning, have grad-
ually widened or terminated as large concentrations of black shape-
less masses. These masses are generally located a short distance
below the crust, but some have been observed in the deeper portion
of the interior. The latter might have had a different mode of
origin and existed prior to the meteorite's entrance into the earth's
atmosphere.

A distinct feature which the vein material exhibits is the presence
of small, rounded troilite bodies, generally along the margins of
the veins. The marginal location of these bodies suggests that
they have been forced out by a viscous medium that rejected these
heavy insoluble grains, and they solidified along the cool borders.

644 FIELDIANA: GEOLOGY, VOLUME 10

Globular troilite bodies, however, are not confined to the margins
of the black veins. They may occur as isolated inclusions which
vary in size; the largest ones are generally oval or elliptical with
their long axes parallel to the veins. More remarkably, troilite
may occur as delicate threads that occupy the center of the black
veins. These threads are interspersed with minute troilite globules,
isolated or in clusters. Such an arrangement simulates an irregularly
strung string of beads. Besides occurring in the various forms re-
ferred to here, troilite may occur as flakes and plates both in the
black veins and in the lighter-colored groundmass but more abun-
dantly in the latter, in which the spherical or oval-shaped troilite
bodies are extremely rare.

The majority of the troilite bodies contain inclusions of nickel-
iron (kamacite), even though the two compounds are immiscible
and the crystallization temperature of the two is vastly different.
The association of the two unlikes represents a eutectic between
the two phases, Fe-FeS, the eutectic temperature being 988° C.
at one atmosphere. It should be recalled, however, that the stone
had suffered brecciation, a process which required much higher pres-
sure to be effective. As such, the eutectic temperature was also
higher than indicated under one atmosphere pressure. The kamacite
plates are intergrown with the troilite bodies and have no set
arrangement; they present a cuneiform appearance or graphic
texture, being the result of simultaneous crystallization of the
two minerals.

The production of an elaborate vein system in the groundmass
of meteorites, such as in this one, presents a subject which cannot
be satisfactorily dealt with from studies of a few examples. We
have, therefore, restricted our studies to the Walters stone and
noted what we have observed. We have been, however, substantially
aided by the previous studies of the Paragould meteorite, in which
the veining system is somewhat similar in distribution but more
intricate.

Extensive veining implies extensive brecciation. The more severe
the brecciation, the more numerous are the veins, for veins are
filled-in cracks or fissures between and around fragments of brec-
ciated meteorites. The filled-in vein matter is not matter intro-
duced into cracks except in polymict meteorites, which are the rarest
among meteorites. As a rule, veins have been formed in place by
thermo-metamorphism of the substance of the meteorite. Extensive
or even moderate brecciation can hardly take place either from shock

ROY, GLASS, AND HENDERSON: WALTERS METEORITE 545

or from pressure during a meteorite's flight or from the impact
with the earth.

The vein system of the Walters meteorite thus arouses serious
doubt that the cracks were formed by any of the methods cited
above. Cracks formed in this manner are not likely to be filled
to form veins by the penetration of heat into the supposedly cold
interior during a few seconds of terrestrial flight of the meteorite.
It is more likely that the Walters meteorite was severely brecciated
earlier in its cosmic history, prior to the disruption of the parent
body, and that many of these veins were produced by hot gas that
penetrated into cracks and fused the constituents of the walls
of the cracks. The linear alignment of the troilite bodies, formed
from reaction of sulphur in the vapor and nickel-iron, suggests
flowage. Apparently, the mass was heated above the melting point,
and this rendered the molten material sufficiently fluid to flow.
The molten matter may have been injected into the fissures during
one time or another of the meteorite's metamorphic history, thus
forming some of the veins. There is evidence that the vein system
of the Walters meteorite was not completed in only one stage,
by the penetration of hot gas into the cracks. It is also clear that
some of the veins that are contiguous to the crust were formed from
fused surface matter that flowed into fissures during the meteorite's
terrestrial flight. The heat encountered during the flight could well
have melted away some of the pre-existing vein material from the
fissures that adjoined the surface of the meteorite. The opening
up of these fissures would have allowed the fused surface matter
to flow into them and form veins of a later generation, which would
be close to the surface and due to an excess of molten matter would
be generally thicker and wider. That many of these marginal veins
are distinctly shorter and wider and that they abruptly thin out
and connect the narrower veins of the interior lend support to
this view of their origin.

Reference has been made to the vein system of the Paragould
stone as being more intricate than the present one. By this, it
is meant that Paragould has passed through one or more additional
cycles of thermo-metamorphism; that is, it has undergone further
crushing, melting, and consolidation, after its major metamorphic
features were developed. This is strongly indicated by the small an-
gular fragments of black material that are enclosed in the gray matrix.
Some of these fragments are isolated, and there are no visible con-
necting veins or passageways through which the black material

546 FIELDIANA: GEOLOGY, VOLUME 10

might have been injected. The presence of black chondrules en-
closed in fused and unfused groundmass, of olivine and enstatite
chondrules within a black matrix, and of chondrules with fused
matter at center, may be considered as further evidence of an
additional cycle of metamorphic alterations suffered by the Para-
gould stone.

The stone is chondritic, as may be inferred from the reference
already made to chondrules which have been cut across or sur-
rounded by veins. Of the number of thin sections examined, few
show well-defined chondrules. The majority of them are either de-
formed or broken. Some are so badly crushed that they can hardly
be distinguished from the groundmass. The total effect of the
deformation, fragmentation, and crushing lends to the meteorite
the appearance of a howardite. Well-defined chondrules are generally
very small in size and are composed of aggregates of olivine grains.
No entire larger olivine chondrules of any type have been observed.
They are chiefly represented by fragments composed either of olivine
bars, or of a mixture of olivine and hypersthene lamellae, or merely
of crystalline aggregates of these two minerals. Scattered grains
of glass and feldspar in some of these chondrules and in the groimd-
mass are of common occurrence. The hypersthene chondrules are
generally fibrous and are characterized by multiple centers of crystal-
lization. Eccentrically radiating hypersthene chondrules were looked
for but none was found, nor did we find a single well-defined glass
chondrule. In a meteorite so highly metamorphosed, the absence
of glass chondrules seems unusual, although glass is one of the
common constituents of this meteorite. Feldspar, both fragmental
and twinned, has been detected, especially in the groundmass of
many of the sections examined.

CHEMICAL AND MINERALOGICAL COMPOSITION

The analytical investigations on this meteorite were made to
establish the composition of the light- and dark-colored portions
and to find how uniform they were. Two samples were prepared
from the light and dark areas for chemical analyses. The material
was dissolved in dilute hydrochloric acid and divided into two por-
tions, the acid soluble material and the insoluble residue.

When the acid attacks the powdered sample the olivine is rapidly
decomposed and some silica separates out. When this silica encloses
some of the unattacked powder it seriously interferes with the com-

ROY, GLASS, AND HENDERSON: WALTERS METEORITE 547

Analysis of the Acid Soluble Portion

Insoluble 43.15

SiOj (soluble) 18.71

MgO 18.40

FeO 11.81

A1,0,
Fe,0,

P«05.

CaO.

Fe..

S...,

Ni...

Co...

Light-Colored
1 2

41.85
19.42
18.85
12.69
0.32
0.01

Ratio

SiOi. .
AI2O3
FejOj
PjOb.
FeO..
CaO.
MgO.
FeO..

Si02
MgO

0.47

0.66

n.d.

0.25

3.65

2.09

1.01

0.31

n.d.

n.d.

1.03

Dark
1

33.51

21.71

19.32

17.31
1.01
1.98
0.25
0.68
2.39
1.37
0.57
0.017

1.12

Veins

2

39.10

19.59

17.52

15.46

0.58

3.42

n.d.

0.47

2.34

1.33

n.d.

n.d.

1.11

Analysis of the Insoluble Material

it-Colored

Dark Veins

56.14

55.90

5,23

4.23

1.39

1.11

n.d.

7.91

7.10

3.60

3.50

19.32

20.92

0.52

0.35

plete digestion of the sample in the acid. Also, an appreciable
proportion of the silica which should belong to the olivine con-
taminates the insoluble material.

For the above reasons one rarely gets satisfactory checks to
the determinations even when the sample analyzed came from the
same tube. In these analyses, the insoluble residue was filtered
off and treated with sodium carbonate to dissolve the silica from
the oh vine; then the residue was again treated with hydrochloric acid.

The treatment of the sample with sodium carbonate possibly
may not have contaminated the insoluble residue for an analysis
of the alkali metals, but since this was not positively known it
seemed best to omit alkali determinations.

SPECIFIC GRAVITY

Before the material was treated with acid a series of density
measurements was made on the selected areas. The density of the

648 FIELDIANA: GEOLOGY, VOLUME 10

lighter-colored portion was 3.52, 3.53, 3.52 and 3.54, while that
of the darker areas was 3.58, 3.56, 3.56. Thus in all cases the material
filling the veins is slightly heavier than the lighter-colored matrix.

MINERALOGICAL COMPOSITION

The following are the non-opaque minerals identified optically.
In order of abundance:

Olivine (Chrysolite). — Olivine is the predominant mineral. It forms
a large part of the coarse-grained groundmass and occurs in chon-
drules as swarms of small grains with random orientation, or as
bars and lamellae having the same orientation.

Much of the olivine has become stained through the oxidation
of the iron. The mineral grains are brown or reddish, and some are
blackish and opaque; only a few grains have clear, pale, grayish-
yellow color.

Optical properties: The mineral is negative. The optic axial
angle is large, (-) 2V=85°-88°; dispersion distinct, r>v. The in-
dices of refraction are: a= 1.680, i3 = 1.701, 7 = 1.720, B.=0.040.

Hypersthene (Bronzite) (Eusa). — Like the olivine with which it
is often intergrown, the hypersthene is stained reddish brown, but
it can be cleaned easily by acid. It occurs as rounded grains and as
fibers; in chondrules it is prismatic to fibrous.

Optical properties: The optic axial angle is large, (-) 2V=80''.
Faint traces of lamellae twinning. The indices of refraction are
a = 1.675, ^=1.682, 7 = 1.686.

Apatite (Manganapatite) . — A relatively abundant mineral found
in clear, usually colorless grains throughout the groundmass corres-
ponds to manganapatite. Grains isolated and tested reacted for
phosphoric acid and manganese.

Optical properties : Uniaxial negative, e = 1.652, o) = 1.657,B.= 0.005.

An apatite mineral which has similar properties was described
as chlorapatite by Larsen and Shannon (Amer. Jour. Sci., 209,
p. 250, 1925). The indices of refraction of this mineral, however,
are higher than those for that chlorapatite.

Merrillite. — It occurs in less abundance than manganapatite.
Optical properties: Uniaxial negative. e = 1.620, 00 = 1.623.
AnorthiteC^) . — A high temperature feldspar.
Optical properties: Biaxial negative. (-) 2V= 40-55° (var.). Shows
pseudohexagonal twinning, and, very rarely, traces of lamellar twin-

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560 FIELDIANA: GEOLOGY, VOLUME 10

ning. Inclusions of high-index minerals are common. Indices:
a = 1.533, /3=1.538, 7=1540. B.=0.007.

Glass. — One of the common constituents of this meteorite. It
is distributed throughout the groundmass, in the veins, and in
the chondrules. Some is clear and colorless, but much of it is stained
and clouded with inclusions.

Optically the clear transparent material is isotropic with n= 1.505.

Percentage determinations of the constituent minerals other than
the order of their abundance have not been made.

X-ray diffraction pattern shows the presence of the following
minerals: taenite, kamacite, olivine, hypersthene or bronzite, pla-
gioclase of undeterminable composition, and troilite.

It will be seen that the results of X-ray studies conform closely
with those obtained by other methods. The mineralogical composi-
tion itself is also much the same as that of certain other chondrites.
The meteorite, however, possesses certain features which may be said
to be distinctive. One of these is the presence of the numerous
troilite bodies, many of which are intimately mixed with kamacite;
the other is the abundance and extent and the nature of the dis-
tribution of the black veins. Apparently, both of these features were
developed in consequence of the heat produced during brecciation
and metamorphism, to which the Walters meteorite was, doubtless,
subjected early in its cosmic history.

REFERENCES

Roy, S. K., and Wyant, R. K.

1955. The Paragould meteorite. Fieldiana: Geol., 10, no. 23, pp. 283-304.

Wahl, W.

1952. The brecciated stony meteorites and meteorites containing foreign frag-
ments. Geochim. et Cosmochim. Acta, 2, pp. 91-117.